For absorbent articles of the above mentioned kind it is of essential importance that they have the ability to quickly receive and absorb large amounts of liquid. It is further of importance that liquid can be distributed from the wetting area to other parts of the absorbent structure, so that the total absorption capacity of the article can be utlilized.
It is further essential that the absorbent structure can retain the liquid also when exerted to external loadings, such as when the user is moving, is sitting or lying down.
One problem, especially for diapers and incontinence guards, which are intended to receive and absorb relatively large amounts of liquids, is that they risk to leak before their total absorption capacity is fully utilized. A reason for such a leakage is that the absorbent structure, especially at repeated wettings, has a decreased capacity to quickly receive and absorb large amounts of liquid.
Absorbent articles of this kind comprise a liquid pervious topsheet, a liquid impervious backsheet and an absorbent structure arranged therebetween, said absorbent structure comprising a liquid acquisition layer located adjacent the liquid pervious topsheet, and one and more liquid storage and liquid distribution layers.
In order to obtain a high absorption capacity and a high liquid retaining capacity when the article is exerted to outer loadings it often contains so called superabsorbents. Superabsorbents are crosslinked polymers with capacity to absorb liquid in an amount several times, 10 times or more, their own weight. They further have the capacity to retain absorbed liquid also when exerted to an external pressure. They have gained wide use in absorbent articles, at which they usually are in particulate form, such as grains, granules, flakes or fibers and they are mixed or layered with other absorption materials, usually cellulosic fibers.
The efficiency of a superabsorbent is dependant on many factors, such as where and how the superabsorbent is mixed into the absorbent structure, what physical shape the superabsorbent particles have, and the properties of the superabsobent such as absorption rate, gel strength and liquid retaining capacity.
An important reason that the absorbent structure functions unsatisfactory at repeated wettings, i e at the second and third wetting occasions, is that it is difficult for the superabsorbent material to maintain its structure and shape after the superabsorbent particles have swollen. The firmness and shape of the superabsorbent particles can for example be weakened at outer loadings. By the fact that a superabsorbent at external loadings and after the first and second wetting has difficulty to maintain its structure and shape, a common phenomena called gelblocking occurs. Gelblocking means that the superabsorbent when wetted forms a gel which blocks the pores in the porous fiber structure and by that detonates the liquid transport from the wetting area to other parts of the absorbent structure. This involves that the total absoption capacity of the absorbent structure is not optimally utilized, and also involves an increased risk for leakage.
The problem of gelblocking increases when the amount of superabsorbent material in an absorbent structure is high. In order to make an article that is discrete and comfortable to wear it is however a desire that the article is thin. In order to maintain a high liquid absorption capacity such thin articles often have a relativley high amount of superabsorbent material.
In order to improve the capacity of the superabsorbent to maintain its structure even at an outer loading and after a plurality of wettings the superabsorbent material is often crosslinked in two steps. The first crosslinking is a so called internal crosslinking and is made by copolymerization of acrylic acid and at least one bifunctional agent under the formation of a network.
Copolymerizeable crosslinking agents used in superabsorbent polymers usually consist of bifunctional substances such as diacrylate esters and allylmethacrylates of trifunctional substances such as 1,1,1-trimethylol propane triacrylate and triallylamine or of tetra-functional substances such as tetraallyloxyethane.
The second crosslinking is a so called surface crosslinking and involves that the superabsorbent easier maintains its original shape also when exerted to external loads and after several wettings. Surface crosslinking of the superabsorbent is usually made by esterification of carboxylic groups. One example of surface crosslinking agents are polyhydroxy substances. Another example is organic carbonates, preferably ethylene carbonate in acqeous solution. A third example is the use of diglycidyl compounds, especially ethylene glycol-diglycidylether (EDGE).
It is also known through e g U.S. Pat. No. 4,043,952 to surface crosslink a superabsorbent based on an anionic polyelctrolyte with a polyvalent metal ion, for example aluminum. The surface crosslinking occurs with ionic bonds. It is stated that the superabsorbent in question has an improved dispersability in an acqeous medium. There is no mentioning about an effect on the absorption capacity in an absorbent article.
Through EP-B-0 248 963 it is known to surface crosslink a superabsorbent of anionic character with a polyquartenary amine for increasing the absorption capacity of the superabsorbent. Also here the crosslinking is made by means of ionic bonding.